For some time paper makers have sought ways to make tissue papers which are soft, yet have sufficient strength.
U.S. Pat. No. 3,556,931 describes treating a sheet of paper with a quaternary ammonium salt debonding agent to soften the sheet. The debonding agent is sprayed on the sheet prior to passing the sheet through a drier.
U.S. Pat. Nos. 4,351,699 and 4,441,962 describe the addition of a quaternary ammonium compound, and at least one specified nonionic surfactant into an aqueous papermaking furnish for making soft, absorbent products such as paper towels. The addition of only quaternary ammonium debonding agents is said to enhance softness, but will also decrease absorbency. The nonionic surfactants are added to overcome the problem of reduced absorbency.
U.S. Pat. No. 4,940,513 describes treating tissue paper with a noncationic surfactant to impart softness. The noncationic surfactants are said to include anionic, nonionic, ampholytic and zwitterionic surfactants. The noncationic surfactants are preferably sprayed on the wet tissue web as it courses through the papermaking machine.
U.S. Pat. Nos. 5,217,576; 5,223,096; 5,240,562; 5,262,007; and 5,279,767 describe the use of quaternary ammonium compound debonding agents for softening tissue paper. Anionic surfactants are described as optional ingredients which can be added to the papermaking furnish so long as they do not significantly and adversely affect the softening, absorbency, and wet strength enhancing actions of the required chemicals.
U.S. Pat. No. 5,494,731 describes tissue papers which have been treated with certain nonionic softeners. The background portion of this document describes certain disadvantages of using cationic debonding agents.
There are numerous problems we have observed with available tissue papers. For example, softness and strength are two important attributes of tissue and towel products. Typically, however, one of those attributes is enhanced at the expense of the other.
One effective technique for enhancing the softness of tissue and towel products is the addition of cationic softeners or debonders to the fiber furnish from which the tissue or towel is made at the wet end of the papermaking system. Unfortunately, the addition of cationic debonders to fiber furnish at the wet end often results in significant reduction of tensile strength (e.g., 15-50% depending on amount added and point of addition). Usually, the furnish, to which debonders are added, is then subjected to refining or the addition of dry strength additives to negate the strength reduction that occurs because of debonder addition. Such treatments, however, often negate the softness benefits imparted by debonder addition. Depending on the type of debonders added, the absorbency rate of the tissue and towel products can also be decreased because of the hydrophobic groups associated with the various debonder formulations.
Cationic debonders, because of their positive charge, are retained on the fiber. On the other hand, anionic softeners and surfactants, because they have the same charge as the fiber, are not sufficiently retained on fiber when they are added to the wet end of the papermaking process. As such, they typically do not function effectively as softeners. They do, however, contribute to wet-end deposition and significant foaming that is detrimental to paper machine operation.
Accordingly, it is an object of the invention to provide a tissue paper product of improved softness, strength and absorbency. It is also an object to provide a manufacturing process for such a tissue paper product where the generation of foam is reduced, or eliminated altogether.